The Internal Revenue Code (USC Title 26), the main tax law in the United States, is in severe need of reform.
It is currently nearly *four billion* words long, over 500,000 lines, and thus entirely impossible to comprehend without years of training and study.
This fact, in and of itself, gives the tax system a bias for the rich and corporations, who can afford experts to find every possible loophole to reduce their personal tax burden.
Any fundamental changes can come later. For now, simply reducing the complexity of the tax code without changing the amount of taxes paid by each income bracket would be a good start, and one that is unlikely to be politically impossible.
Make it simple enough that a regular citizen can do his own taxes without missing thousands of potential tax breaks.